Investigating Science 11–12 · Year 11

The Bernoulli effect measured in a Venturi tube

Module 1: Cause and Effect – Observing

Practical, model not builtLow risk

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The idea

Where a flowing fluid speeds up through a narrow section its pressure falls, which a pair of water columns shows as a measurable height difference that Bernoulli’s equation predicts.

What you need

a clear horizontal Venturi tube narrowing from 20 mm to 10 mm internal diameter, with vertical pressure tubes at the wide and narrow sections (a laboratory Venturi apparatus or a teacher-built version)
hose from a tap with a flow regulator, a 1 L measuring cylinder and a stopwatch to measure the flow rate
ruler taped behind the pressure tubes, food colouring, a tray under the outlet
for the qualitative stations: a paper strip, a table-tennis ball and a hair dryer on its cool setting

How to do it

Run water through the tube at a steady rate and time how long it takes to collect 1.00 L; repeat three times to find the flow rate Q.
Read the water heights in the wide-section and narrow-section tubes to the nearest millimetre.
Repeat at four other flow rates.
Calculate v₁ = Q/A₁ and v₂ = Q/A₂ and the predicted height difference Δh = (v₂² − v₁²)/(2g).
Plot measured Δh against Q²; a straight line through the origin supports the model.
At the qualitative stations record what happens when air is blown over the paper strip and when the ball sits in the air stream, and list which statements are observations and which are explanations.

What you should see

At Q = 6.0 L/min (1.00 × 10⁻⁴ m³/s) the water moves at 0.318 m/s in the 20 mm section and 1.27 m/s in the 10 mm throat, and the pressure tube at the throat stands 7.75 cm lower (a pressure difference of 758 Pa). Δh grows with Q², so doubling the flow quadruples the difference; measured differences can be a little larger than the ideal value because friction between the two tappings adds a head loss. The paper-strip and floating-ball stations give clear qualitative observations, but their explanation involves more than Bernoulli’s equation, which is why the measured Venturi is the quantitative test.

What changes

What you change: flow rate Q
What you measure: height difference between the pressure tubes
What you keep the same: same tube and tappings
steady flow before reading
water temperature
three timings per flow rate

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Faster-moving fluid pushes harder, so its pressure is higher.
Every lift or suction effect in moving air is explained by Bernoulli’s equation alone.
The pressure falls because the narrow tube squeezes the water.

Safety card

Low riskLearners carry it out

Hazards

water on the floor
hair dryer near water

Controls

work over a tray and mop spills
keep the hair dryer at a separate dry station

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-8 INS11/12-1 INS11/12-3 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities